An Earthcache is not just for Christmas! - The River Colne - Sedimentary processes in action
This Earthcache is all about sedimentary processes occurring in the River Colne.
When sediment (sand, gravel, clay, etc) enters the River Colne, one of two things is likely to happen to it. The sediment will either sink to the bottom and stay there or it will be carried along in some way by the water.
Imagine stirring a bucket of water and that, as you stir, you drop a handful of sand and pebbles into it.
Which factors will determine whether the sand and pebbles fall to the bottom of the bucket, motionless, or are swept round the bucket by the moving water?
1. How fast the water is moving (its speed).
2. How big the particles are (their grain size).
3. The density of the particle.
4. The shape of the particle.
The faster the water is moving, the smaller the particle, the smaller the difference in density between the sediment and the water, and the flatter or flakier the particle, the more likely it is to be swept round in the water in the bucket.
The same is true in the river beneath your feet!
The co-ordinates take you to this spot.
Some of the sediment in the River Colne will be swept along with the water. The largest particles will roll or slide across the river bed whereas smaller particles will bounce along, knocking into each other and ricocheting upwards. This bouncing mode of travel is called saltation (from the Latin saltare, ‘to leap’). Together the rolling, sliding and saltating grains make up what is called the bedload of the moving sediment, because they are continuously or intermittently in contact with the bed during transport. The finest particles are caught up in turbulent eddies and carried in suspension and never touch the base of the river. These form the suspension load of the moving sediment.
To work out how different sediment may be transported by the river we need to know the speed of the water.
To claim this Earthcache you must estimate the speed of the water at this section of the River Colne and therefore determine which sediment may be transported by the water.
The speed of the water can be estimated by playing ‘Pooh sticks’! I have added a team work attribute to this cache as this section of the road can be very busy.
Please do not attempt this cache on your own!!!
Time how many seconds it takes for your ‘Pooh stick’ to travel under the bridge, from one side to the other. The distance across the bridge is approximately 10 metres. To find the speed of the water current use the following formula:
Speed = Distance / Time
e.g. On 18th November 2012 my Pooh stick took 15 seconds to travel under the bridge.
So 10 metres / 15 seconds = 0.667 metres per second
Looking at the graph below, shows that at this speed clay, sand, gravel and some small pebbles will be carried along in the water.
The red line shows the speed of the water on the day I visited. Use the graph to work out which sediment is being transported on the day you visit.
Please email me the answers to the following questions to claim this Earthcache:
1. How many seconds did it take for your 'pooh stick' to travel under the bridge?
2. What was the speed of the water on the day you visited?
3. Using the graph above which grains of sediment may have been transported? Clay, Silt, Sand, Pebbles?
4. Looking at the river bed you can see pebbles. What shape are these pebbles? And why might that be?
In keeping with recent changes to Earthcache rules, photographs are now optional and I cannot require these to be posted. A photo speaks a thousand words though, so if you have a nice photo of any combination of you, your GPSr, the bridge and the river I hope you will choose to post one to the cache page anyway!
Please email me the answers to questions 1, 2, 3 and 4 and post your photo in your log. Please do not post the answers to questions 1, 2, 3 and 4 in your log. Logs with this information in will be deleted.